1. Field of the Invention
The present invention relates to ultrasonic densitometer equipment using ultrasonic sound waves to measure bone integrity, and in particular, to an ultrasonic densitometer in which the ultrasonic signal is coupled to a human heel through liquid filled bladders.
2. Description of the Prior Art
Various devices presently exist which may be used to measure the physical properties and integrity of a member such as a bone. Non-invasive density measuring devices can be used to determine cumulative internal damage caused by micro-crushing and micro-fracturing occurring in the bones of humans or animals such as racehorses. Additionally, osteoporosis, or loss of bone mineralization, detection in humans and its cure or prevention are increasingly becoming areas of intense medical and biological interest. As the average age of the human population increases, a greater number of patients are developing complications due to rapid trabecular bone loss.
Early Work
U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuring the density of a bone structure, such as a finger bone or heel bone, to monitor the calcium content thereof. The device includes a pair of opposed, spaced ultrasonic transducers which are held within a clamping device clamped on the bone being analyzed. A pulse generator is coupled to one of the transducers to generate an ultrasonic sound wave which is directed through the bone to the other transducer. An electric circuit couples the signals from the receive transducer back to the pulse generator for retriggering the pulse generator in response to those signals. The pulses therefore are produced at a frequency proportional to the transit time that the ultrasonic wave takes to travel through the bone structure which is directly proportional to the speed of the sound through the bone. The speed of sound through a bone has been found to be proportional to the density of the bone. Thus the frequency at which the pulse generator is retriggered is proportional to the density of the bone.
Another device and method for establishing the in vivo strength of a bone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 to Pratt, Jr. The device includes a launching transducer and a receiving transducer, which are connected by a graduated vernier and which determine the speed of sound through the bone to determine its strength. The vernier is used to measure the total transit distance between the surfaces of the two transducers.
Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue, Tissue Characterization With Ultrasound, CRC publication 2, pp. 207-226) discusses various studies involving attenuation and speed of sound measurements in both cortical and spongy (cancellous or trabecular) bone. The results of these studies reveal a linear relationship between the wet sonic velocity and wet cortical density, and between the dry sonic velocity and the dry cortical density. The transit times of an acoustic signal through a bone member therefore are proportional to the bone density. Langton, et al. (Langton, C. M., Palmer, S. D., and Porter, S. W., (1984), The Measurement of Broad Band Ultrasonic Attenuation in Cancellous Bone, Eng. Med., 13, 89-91) published the results of a study of ultrasonic attenuation versus frequency in the os calcis (heel bone) that is utilized through transmission techniques. These authors suggested that attenuation differences observed in different subjects were due to changes in the mineral content of the os calcis. They also suggested that low frequency ultrasonic attenuation may be a parameter useful in the diagnosis of osteoporosis or as a predictor of possible fracture risk.
Current Devices
While it has long been recognized that there is a general relationship between the propagation of ultrasonic energy through material and the strength and condition of the material, clinical success of an ultrasonic densitometer requires repeatable and accurate measurements that may be directly related to bone quality for a given individual.
Important in reproducible and accurate ultrasonic measurement is obtaining sufficient signal strength at the receiving transducer to overcome the effects of noise in the measurement and properly restraining the patient limb.
High signal strength may be obtained by appropriate acoustic couplants that provide a low impedance acoustic path between the transducers and the patient. The couplant should conform to the irregular surfaces of the human heel or other member to prevent air pockets or the like.
One method of providing such coupling is described in U.S. Pat. No. 4,930,511 to Rossman issued Jun. 5, 1990 and assigned to the assignee of the present invention. The Rossman patent describes a pad of urethane or other compliant material that may form a bridge between the transducer and the patient""s foot.
Alternatively as taught in U.S. Pat. No. 5,042,489 to Wiener issued Aug. 27, 1991 and also assigned to the assignee of the present invention, the patient""s foot may be surrounded by a waterbath providing for a gapless and continuous liquid bridge between the patient""s foot and opposed transducers.
A combination of these approaches uses a compliant plastic membrane filled with a liquid such as is taught in U.S. Pat. No. 5,134,999 to Osipov issued Aug. 4, 1992; U.S. Pat. No. 5,772,596 to Forfitt et al. issued Jun. 30, 1998; and U.S. Pat. No. 5,817,018 issued Oct. 6, 1998 to Ohtomo. The Ohtomo device and the Osipov device use sealed bladders while the Forfitt device allows for inflation of the bladders against the patient. In each of these devices, the limited compliance of the bladder and the relatively large gap required for the insertion of the foot, require the bladders to be mounted on a movable support that may be moved close to the foot after the foot is in position.
As mentioned, it is also important that the patient""s limb be properly immobilized to reduce movement artifacts and so that multiple measurements of the same portion of the bone may be made and averaged or so that more sophisticated techniques, such as scanning and identification of a region of interest (ROI) fixed with respect to the patient""s bone may be used.
U.S. Pat. No. 5,014,970 to Osipov issued May 14, 1991 describes a device for immobilizing a patent""s foot for ultrasonic measurement employing bladders inflated with air to provide a clamping at a controlled pressure downward upon the patient""s foot. The bladders do not touch the foot but are held within a relatively complex clamping structure including an overarching bridge which straddles the patient""s foot and a shell conforming to the upper surface of the patient""s foot having a foam lining to prevent uncomfortable pressure points.
While methods are known to provide acoustic coupling and restraint of the patient limb, they are complex and require multiple structures. What is needed is a relatively simple method of providing an acoustic path between transducers and the patient member and of immobilizing that member for measurement.
The present inventors have recognized that the expandable bladders known in the prior art for providing a liquid filled coupling path between an ultrasonic transducer and the patient, if properly designed, can serve also to immobilize the patient""s limb precisely at the point where imaging is to be performed. This stabilization is accomplished by enlarging the base of the bladder and limiting its expansion to that approximating a hemisphere. The shallow inflation of the bladder provides improved lateral stiffness to hold the patient. Increasing the size of the base of the bladder allows substantial distances to be spanned within this shape constraint. In this way, patient restraint may be incorporated into the same mechanism used to provide acoustic coupling between the transducers and the patient. Simple inflation of the bladder eliminates the need for complex auxiliary methods of moving the bladders toward or away from the patient.
Specifically then, the present invention provides an ultrasonic densitometer for measuring bone character and including a first and second ultrasonic transducer mounted in opposition along a sound path about a patient""s support. At least one flexible membrane is provided holding an acoustic coupling fluid within a volume in contact with the first transducer. The flexible membrane is mounted at its periphery to a base fixed with respect to the patient""s support to present a flexible membrane surface facing the second transducer along the sound path and having a first shape providing an unobstructed gap along the sound path sized to receiving a human limb. A pump inflates the flexible membrane by an extension distance out along the sound path from the periphery to assume a second shape reducing the unobstructed gap to press the flexible membrane surface against a human limb if placed within the gap. The extension distance is less than the diameter of the flexible membrane periphery.
Thus it is one object of the invention to provide for a method of immobilizing the human limb as well as providing an acoustic coupling path to an image site. By limiting the extension distance with relationship to the base, a laterally stabilizing surface is produced contrasting with highly cantilevered bladders used in the prior art.
It is another object of the invention to provide for a stabilization mechanism that provides for the greatest stabilization exactly at the imaging site. By using the membranes both for acoustic coupling and stabilization, stabilization specific to the imaging site may be achieved.
The pump may be a rolling diaphragm pump.
Thus it is another object of the invention to provide for the elimination of sliding seals and the like such as may create leakage in an inflated-type system and further to provide for a pump that allows precise volumetric control.
The device may include a reservoir holding acoustic coupling fluid prior to inflation of the flexible membrane and further a heater controlling the temperature of the acoustic coupling fluid in the reservoir to a predetermined temperature.
Thus it is another object of the invention to provide a method of providing temperature-stabilized water to a membrane system overcoming the fact that heating devices may not easily be inserted into the membrane volume.
The first and second acoustic transducers may be fixed in separation.
Thus it is another object of the invention to provide for an acoustic coupling and limb immobilization method accommodating the large gaps required for fixed transducer designs.
The device may include a pressure transducer communicating with the acoustic coupling fluid and the pump to control the pressure of the coupling fluid in the flexible membrane.
Thus it is another object of the invention to provide for a precisely controlled pressure against the foot such as may improve repeatability and measurements.
The device may include a second flexible membrane holding an acoustic coupling fluid within a volume in contact with the second transducer, the second flexible membrane mounted at its periphery to the base fixed with respect to the patient""s support to present a second flexible membrane surface facing the first transducer along the sound path and having the first shape providing the unobstructed gap along the sound path sized to receive the human limb. The second membrane may communicate with the pump to inflate by the extension distance out along the path from the periphery to assume the second shape reducing the unobstructed gap to press the second flexible membrane against the human limb placed within the gap.
Thus it is another object of the invention to use opposed inflatable membranes to provide an increased unobstructed gap for insertion of the foot.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.